The present invention relates generally to data communication networks and more particularly relates to a method for determining the Round Trip Time between a source end station and a destination end station utilizing the ATM Traffic Management mechanism.
Currently, there is a growing trend to make Asynchronous Transfer Mode (ATM) networking technology the base of future global communications. ATM has already been adopted as a standard for broadband communications by the International Telecommunications Union (ITU) and by the ATM Forum, a networking industry consortium.
ATM originated as a telecommunication concept defined by the Comite Consulatif International Telegraphique et Telephonique (CCITT), now known as the ITU, and the American National Standards Institute (ANSI) for carrying user traffic on any User to Network Interface (UNI) and to facilitate multimedia networking between high speed devices at multi-megabit data rates. ATM is a method for transferring network traffic, including voice, video and data, at high speed. Using this connection oriented switched networking technology centered around a switch, a great number of virtual connections can be supported by multiple applications through the same physical connection. The switching technology enables bandwidth to be dedicated for each application, overcoming the problems that exist in a shared media networking technology, like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI). ATM allows different types of physical layer technology to share the same higher layerxe2x80x94the ATM layer.
ATM uses very short, fixed length packets called cells. The first five bytes, called the header, of each cell contain the information necessary to deliver the cell to its destination. The cell header also provides the network with the ability to implement congestion control and traffic management mechanisms. The fixed length cells offer smaller and more predictable switching delays as cell switching is less complex than variable length packet switching and can be accomplished in hardware for many cells in parallel. The cell format also allows for multi-protocol transmissions. Since ATM is protocol transparent, the various protocols can be transported at the same time. With ATM, phone, fax, video, data and other information can be transported simultaneously.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol to the ATM switch. ATM provides the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
The current standard solution for routing in a private ATM network is described in Private Network to Network Interface (PNNI) Phase 0 and Phase 1 specifications published by ATM Forum. The previous Phase 0 draft specification is referred to as Interim Inter-Switch Signaling Protocol (IISP). The goal of the PNNI specifications is to provide customers of ATM network equipment some level of multi-vendor interoperability.
The Interim Local Management Interface (ILMI) for the PNNI protocol specification provides an auto-port configuration capability. This capability functions to minimize manual configuration operations for PNNI ports of switches. The Phase 0 solution to auto-port configuration is based on hop by hop routing utilizing a xe2x80x98best matchxe2x80x99 scheme. The Phase 1 PNNI based solution is based on Open Shortest Path First (OSPF) with the additions necessary for ATM. This scheme is essentially a xe2x80x98source routingxe2x80x99 scheme whereby each node has basic knowledge of the structure of the entire network and uses this knowledge to build a complete path from the source to the destination. When a connection is to be set up from a source to a destination, the source sends out a SETUP message that has within it the address of the destination. Each ATM network node along the way reads the next node from the SETUP message and forwards the message to an appropriate next node. This continues until the SETUP message arrives at its destination.
In the IISP Phase 0 specification standard, the ATM nodes in the network route the signaling SETUP message hop by hop (i.e., node by node) using a xe2x80x98best matchxe2x80x99 scheme. ATM addresses are 20 bytes long but only 19 bytes can be used for routing purposes. According to the IISP Phase 0 standard, several prefixes of the ATM address for each link can be registered.
When a node (i.e., an ATM switch) needs to decide to which particular node to route the received SETUP message to, it compares the destination address with all the registered addresses for all of its ports. Only if an address prefix is found that fully matches the destination address can the destination address be considered for routing. After all the prefixes are compared, the prefix address that is the longest is used to determine the routing of the SETUP message. It is important to note that the standard does not require the transfer of any routing information between two neighboring nodes. In addition, the standard also does not permit the use of a TRANSIT NET ID parameter during the signaling phase, which can be used to route to a different routing domain.
A disadvantage of this scheme is that all the prefixes of all neighboring nodes must be registered manually on each of their respective ports. For example, if a port is disconnected from a neighbor and connected to a new neighbor, then the registered addresses must be manually changed in both nodes. This type of network can be termed an absolute static network.
The components of the ATM header consist of the following fields. A generic flow control (GFC) field provides flow control; a virtual path identifier (VPI)/virtual channel identifier (VCI) field allows the network to associate a given cell with a given connection; a payload type identifier (PTI) field indicates whether the cell contains user information or management related data and is also used to indicate a network congestion state or for resource management (i.e., the EFCI state which is encoded in the PTI field); a cell loss priority (CLP) field indicates that cells with this bit set should be discarded before cells with the CLP bit clear; a header error check (HEC) field is used by the physical layer for detection and correction of bit errors in the cell header and is used for cell delineation.
The provisioning of an ATM network connection may include the specification of a particular class of service. The following list the various classes of service currently defined in ATM. Constant bit rate (CBR) defines a constant cell rate and is used for emulating circuit switching (e.g., telephone, video conferencing, television, etc.). Variable bit rate (VBR) allows cells to be sent at a variable bit rate. Real-time VBR can be used for interactive compressed video and non real-time can be used for multimedia e-mail.
Available bit rate (ABR) is designed for data traffic (e.g., file transfer traffic, etc.) and is the class service connected with resource management. The source is required to control its rate depending on the congestion state of the network. The users are allowed to declare a minimum cell rate, which is guaranteed to the virtual circuit by the network. ABR traffic responds to congestion feedback from the network.
Both switches and end stations in the network implement ABR. Binary switches monitor their queue lengths, set the PTI for congestion state (EFCI) in the cell headers, but do not deal with the computation of explicit rate feedback when congestion occurs.
Explicit rate switches compute the rate at which a source end station can transmit and place this information in the explicit rate field in the returning resource management cell. The destination sends one resource management cell for every N data cells transmitted. If the source does not receive a returning resource management cell, it decreases its allowed cell rate. This results in the source automatically reducing its rate in cases of extreme congestion.
In the case when the source receives a resource management cell, it checks the congestion indication flag (CI bit), after which the sending rate may be increased. If the flag is set, then the sending rate must be reduced. After this stage, the rate is set to the minimum of the above and the explicit rate field.
A fourth class of service, unspecified bit rate (UBR), is utilized by data applications that are not sensitive to cell loss or delay and want to use leftover capacity. During congestion, the cells are lost but the sources are not expected to reduce their cell rate.
ATM is a connection oriented transport service. To access the ATM network, a station requests a virtual circuit between itself and other end stations, using the signaling protocol to the ATM switch. ATM provides the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network.
Traffic management (TM) can be defined as the process by which the flow of cells from one device to another within the ATM network is controlled in order to allow for the greatest possible flow rate for the longest possible periods of time. ATM network congestion is defined as a state of network elements (e.g., switches, concentrators, etc.) in which the network is not able to meet the negotiated network performance objectives for the already established connections, resulting in lost cells. ATM layer congestion can be caused by unpredictable statistical fluctuation of traffic flows or fault conditions within the network. The purpose of traffic management is to ensure that users get their desired quality of service. During periods of heavy loads, when traffic cannot be predicted in advance, ensuring quality of service presents a problem. This is the reason congestion control is the most essential aspect of traffic management.
Traffic management refers to the set of actions taken by the network to avoid congested conditions. Congestion control refers to the set of actions taken by the network to minimize the intensity, spread and duration of congestion. These actions are triggered by congestion in one or more network elements. In general, the following traffic and congestion control functions are available within ATM networking.
The ATM standard defines a standard mechanism for the ATM network for indicating congestion states (e.g., setting the EFCI encoded in the cell header) and for indicating cell loss priority for selecting which cell to drop first in case congestion exists. Explicit forward congestion indication (EFCI) is a congestion notification mechanism that the ATM layer service user may make use of to improve the utility that can be derived from the ATM layer. A network element sets the EFCI in the cell header in an impending congested or already congested state. A congested network element can selectively discard cells explicitly identified as belonging to a non-compliant ATM connection and/or those cells with their CLP bit set. This is to protect cells without their CLP bit cleared from being discarded for as long as possible.
For ABR traffic, the ATM forum has defined a traffic management scheme that uses Resource Management (RM) cells to control the traffic rate through the network based in part on the EFCI mechanism defined previously. With reference to FIG. 1, a source end station (SES) 20 inserts forward RM cells into the ATM network 22. The destination end station (DES) 24, upon receiving these forward RM cells, turns them around and sends them back as backward RM cells.
In prior art implicit rate control, if there has been congestion on the forward path (recognized at the DES (FIG. 1) by the EFCI bits of the incoming data cells), a congestion field in the backward RM cell is marked (i.e. set to a xe2x80x981xe2x80x99). The SES 20 receives the backward RM cell and acts upon it. If the congestion field indicates a congestion or if the RM cell is not returned, the sending rate is reduced. When the SES 20 receives a backward RM cell with the congestion field not indicating a congestion, it may increase the sending rate on that particular virtual circuit (VC).
Explicit rate control enhances the implicit rate control by adding an explicit rate field to the RM cell. In this field, the SES indicates the rate at which it would like to transmit. If an explicit rate switch exists in the VC route it may reduce the value in the explicit rate field in the backward RM cells in case of congestion. In this case (the example in FIG. 1) explicit rate for the SES is indicated. The SES upon receiving the RM cells, adjusts its sending rate according to the explicit rate fields.
A block diagram of a portion of a typical ATM edge device 12 (i.e. a device that resides on the outer border of an ATM network) is illustrated in FIG. 2. A data source 15 is shown coupled to a cell generator and scheduler 14 which, in turn, is coupled to a cell data buffer 16 and a physical interface 18. Data source 15 may be any device or system that supplies data to be transported over the physical media, e.g., Token Ring, Ethernet, video conferencing, etc. The function of physical interface 18 is to couple transmit and receive data from the physical media to cell generator/scheduler and traffic management module 14. Cell data buffer 16 functions as a temporary holding memory until cell generator/scheduler and traffic management module 14 has finished processing cells. The module functions to transmit cells at the current transmit rate and modify the transmit rate in accordance with traffic management direction.
A diagram illustrating the fields and their positions in a standard Traffic Management RM cell is shown in FIG. 3. The cell format 40 is shown in increasing byte order starting with byte number one. The ATM header comprises bytes 1-5. The protocol ID comprises byte 6. Byte 7 comprises a direction (DR) bit (forward=0, backward=1), a BECN cell indicator (a 1 if switch generated), a congestion indication (CI) bit (a 1 if congestion present), a no increase (NI) bit (a 1 means do not increase rate), a request acknowledge (RA) bit and a 3 bit reserved field. The explicit cell rate (ER) comprises bytes 8-9. Bytes 10-11 comprise the current cell rate (CCR). Bytes 12-13 comprise the minimum cell rate (MCR). The queue length indicator (QL) comprises bytes 14-17. The sequence number (SN) (an integer number) comprises bytes 18-21. Bytes 22-51 and a portion of byte 52 are reserved. A CRC-10 field occupies the remainder of byte 52 and byte 53.
In order to implement traffic management functions and the handling of RM cells, the prior art approach is to modify cell scheduler 14 to include traffic management functions. The disadvantage of this approach is that this is usually a complex and expensive process. In addition, if traffic management functions are to be incorporated into other different cell schedulers, the process must be repeated for each one.
One of the drawbacks, however, of the ATM protocol is its complexity. The protocol requires the setup of numerous parameters prior to the operation of the network and prior to the establishment of each call connection. The Traffic Management mechanism utilizes transmission descriptors in the performance of its tasks. The descriptors comprise a plurality of parameters that characterize the transmission behavior of the SES and are described below.
The CRIM parameter was previously known as the XRM parameter but is currently referred to now as the missing RM cell count or CRM parameter. The CRM field of the Traffic Management descriptor represents the missing RM-cell count that limits the number of forward RM cells that may be sent in the absence of received backward RM cells. In other words, the CRM parameter determines the number of RM cells that can be lost before the VC transmit rate is decreased. For example, if CRM equals 2 and RM cell #5 is about to be transmitted, but RM cells #3 and #4 have not been received yet, the VC transmit rate should be decreased since if the RM cell #5 is sent it could get lost due to congestion in the network.
Note that almost all the other Traffic Management descriptor parameters, e.g., Nrm, PCR, MCR, etc., can be set with values determined a priori. The CRM parameter, however, is strongly dependent on the network topology, i.e., number of hops, and the devices that are in use in the network. In the prior art, the setup of the CRM parameter utilizes the cooperation of one or more network elements during setup or, in the alternative, the CRM parameter is set a priori to a particular value. Note also that bad or incorrect tuning of the CRM parameter (either too high or too low) can lead to a major reduction in performance when using Traffic Management in ATM networks.
As another example, consider a call connection set up between a source end station and a destination end station over a path that comprises a large number of hops. Suppose the CRM Parameter is set to a default value of 1 and the Nrm field (the maximum number of cells a source may send for each forward RM cell) is set to a default value of 32. Data cells begin to be transmitted into the long VC. The transmitting source end finishes transmitting the 32 data cells and transmits the RM cell.
The source end station then examines its receiver waiting for the transmitted RM cell to return. Assume the transmitted RM cell is not received for a relatively long time due to long RTT (many hops) or due to congestion in the network The RM cell did not get lost but the RTT is too long. Since the CRM parameter is not set to a proper value, the source end station, in response to the long RTT, needlessly reduces the transmission rate that results in a reduction of performance.
One the other hand, if CRM is set to too big a value, it will be discovered too late that one or more RM cells were lost, which also leads to degradation of performance. Note that the Nrm parameter can be set to a value less than 32 for fast response to network congestion with the consequence that more RM cells are generated.
Note also that, in connection with the TBE parameter, some vendors do not implement the software option specified by the TBE parameter, thus the CRM parameter must be set a priori in these products.
The present invention is a method of measuring the Round Trip Time (RTT) of a Virtual Circuit (VC) utilizing the Traffic Management mechanism of ATM. The RTT is measured in real time individually for each new opened VC and/or for existing VC routes in the network. In addition, the RTT measurement method of the present invention can be used to determine the CRM parameter for a VC on an end to end basis, e.g., user to user or LEC to LEC, so as to achieve better performance of the network.
The invention comprises a first embodiment that utilizes the SN field of a RM cell in measuring the RTT and a second embodiment does not utilize the SN field in measuring the RTT. In addition, an application of the method is presented in determining the CRM parameter within the TM descriptor.
The method of the present invention is applicable in networks that implement a Traffic Management (TM) mechanism such as specified by the ATM standard in Traffic Management Specification Version 4.0, incorporated herein by reference. The Traffic Management specifications as formed by the ATM Forum specify an end to end congestion control algorithm, i.e., the implicit portion. Three tables may be used for controlling the transmission rate of an individual virtual circuit: (1) Increase table, (2) Decrease table and (3) Xdecrease table.
In operation, the method of the present invention functions to measure the RTT using these three tables. The source end station transmits a known number of data cells and two RM cells. If the transmission rate decreases, it means that the second RM cell did not arrive on time and probably was lost or delayed indicating congestion in the network. CRM is then increased and the RTT measurement is repeated. If the transmission rate did not decrease, Nrm and CRM are assigned new values and the RTT measurement is repeated.
A binary search algorithm is used to set the Nrm and CRM values for each iteration of the method. Once the values converge, the RTT is calculated by multiplying the number of data cells by the transmit time of a data cell and adding twice the transmit time of a RM cell.
The invention also discloses several applications of the RTT measurement method of the invention. One application is in the automatic setup of the CRM Traffic Management descriptor parameter accordingly. Another application is in the measurement of the RTT between two edge devices. A third application is establishing and maintaining a VC between each LEC and continuously or periodically measuring the RTT utilizing the TM mechanism.
There is provided in accordance with the present invention a method for measuring the Round Trip Time (RTT) of a Virtual Circuit (VC) utilizing the Asynchronous Transfer Mode (ATM) Traffic Management (TM) mechanism, the method comprising the steps of initializing the CRM and Nrm parameters in the TM descriptor, neutralizing the Decrease and Increase tables within the TM mechanism, setting the transmission rate to a first rate value, setting the Xdecrease table entry corresponding to the first rate value to a second rate value, neutralizing the entry corresponding to the second rate value, flushing the VC before each iteration, transmitting a plurality of data cells such that a first Resource Management (RM) cell and a second RM cell are transmitted, increasing the value of CRM if the transmission rate decreases indicating that the second RM cell did not arrive in time due to loss or RTT being longer than the time represented by the time between the first RM cell and the second RM cell, assigning new values to CRM and Nrm if the transmission rate did not decrease and calculating RTT once the CRM and Nrm values converge.
The step of neutralizing the entry corresponding to the second rate value comprises the step of placing the second rate value in the entry corresponding to the second rate value.
The step of flushing the VC may include the step of using an Operation And Maintenance (OAM) cell in loopback mode; using the Sequence Number (SN) field of an RM cell for identification purposes, wherein a single counter is implemented for all VCs; using the Sequence Number (SN) field of an RM cell to confirm whether the VC was flushed by sending and receiving an RM cell having the same SN field or waiting a sufficient length of time such that all residual RM cells are cleared from the VC.
The step of assigning new values to CRM and Nrm comprises the step of utilizing a binary search technique to rapidly converge to final values of CRM and Nrm. The step of calculating the RTT utilizes the following equation:   CRM  =      RTT          Nrm      ·      Cell_Time      
wherein the Cell_Time is proportional to the Peak Cell Rate (PCR) of the VC and TM descriptor. In addition, the method further includes the step of varying the transmission rate of each data cell transmitted between RM cells.